1. Field of the Invention
This invention relates in general to passive responders for position finding systems which comprise at least one microwave antenna and a resonator member and forming a part of a position locating system wherein interrogation signals are transmitted by the train as it passes so as to interrogate and receive a signal from the passive responder as the railway vehicle moves by.
2. Description of the prior Art
Position finding systems comprise interrogators which utilize a given frequency band located in the microwave zone and which periodically emit interrogation signals which are changed in frequency as the railroad vehicle passes passive responders mounted along the railway track. Coded information is stored in the responders by means of tuned resonators which information is received by the interrogating units on the moving train by means of frequency selective reflection. This information may include fixed data about distances from fixed reference points and may also include variable data relating, for example, to travel orders.
There are many ways in which the responders can be arranged along the railway. They might, for example, be mounted above the railway as for example on overhead contact wire terminations or on poles in such a way that the main direction of radiation of the antenna is pointed downwardly. In such an arrangement the antenna cover of the antenna on the moving vehicle can be heated. In this fashion and due to the air flow, the antennas of the responders and the interrogating units can be kept free of snow. Very wet snow dampens the transmission path so much that positive transmission between the position finding devices is degradated to a point where proper operation is not assured. For reasons which need not be explained here, the arrangement of such overhead transponding units is not feasible for technical reasons.
Another arrangement would be to mount the responders on posts located at the side of the railway such that the main direction of radiation of the antennas would always be pointed horizontally toward passing vehicles. In such an arrangement although small amounts of snow may adhere to the antennas, generally wet snow would fall from the antenna due to its heavy weight. However, such an arrangement is not usable under actual railway conditions due to weather factors.
The only approved method of mounting the responders is to mount them in or at the tracks in the gravel of the road bed or on the railroad ties. With such mounting, the antennas of the transponders are pointed directly upward, however, such arrangement allows the transmission path between the responders and interrogators to be destroyed due to heavy snow cover which is maximum with such arrangement.